The present invention relates to a system for detecting defective portion, such as pinholes, black spots, scratches, dust and the like, occurring in data recording portions of an optical recording medium, such as an optical card, optical disk, holographic memory medium or the like. This invention particularly relates to a defective portion detection system capable of detecting defective portions occurring on the substantial surface of the data recording portion during the production process of the medium, and the system does not read the fine pattern for recording data which is formed in the data recording portion. Furthermore, the system of this invention is applicable to the detection of defective portions of industrial products without an optical recording medium if the products have fine patterns and also in the case where the defective portions are overlapped on the fine patterns. The industrial products, for example, are photomasks used in the production of semiconductor devices.
An optical card is an example of the optical recording medium and is generally formed in the size of 85.5 millimeters (mm) long, 54.0 mm wide side and 0.8 mm thick, and this size of card conforms to the standard of International Organization for Standardization (IOS). The data recording portion of the optical card is capable of recording a maximum of 2 mega-bytes of data, and it is possible to extremely increase recording capacity in the same area compared with a magnetic card capable of recording a maximum of 72 bytes of data.
FIGS. 1 to 3 generally show an optical card 1. In FIG. 1, a data recording portion 2 is formed within the specific extent on the card 1. The data recording portion 2, as shown in FIG. 3 in enlarged detail, comprises a recording track 2a being about 7 microns (.mu.m) in width and having pits 3, and a tracking groove 2b being about 3 .mu.m in width and having pitches of 10 .mu.m. The card 1 comprises, as shown in FIG. 2, a substrate 4 having the same shape as the card 1, a first metal layer 5 having the same extent of the portion 2 on a side of the substrate 4 and stable characteristics against laser rays, a second metal layer 6 stacked on the first layer 5 and melted by the laser rays so as to react with a third metal layer mentioned after the third metal layer 7 stacked on the second layer 6 and being a metal-oxide thin layer either coloring or discoloring on the basis of the reaction with the second layer 6 by the laser rays, and a protective layer 8 having the same shape as the substrate 4 and covering the data recording portion 2.
The optical card 1 will happen to have defective portions 9, such as pinholes, black spots, scratches, dust or the like, as shown within a circle C of FIGS. 1 and 3 and in FIG. 2, until the card 1 is completed as a product through some stack steps. The defective portions 9, as shown in FIG. 3, occur on both the recording track 2a as a portion 9a and on the tracking groove 2b as a portion 9b. The defective portion 9b on the groove 2b does not function as a fatal defect to the card 1, but in case the defective portion 9a occupies the greater part of the track 2a over 20 .mu.m in diameter, the card 1 will have a fatal defect as an optical card. The track 2a has the pits 3 for recording data, which are formed by melting the second metal layer 6 by radiation of a predetermined quantity of laser rays. There are three types of optical cards which are a retrievable type, an add-on type, and a rewritable type. The retrievable type card has specific data stored in the recording portion and is only read by readers. The add-on type card is offered to users in a state without any data recorded thereon and is capable of recording the desired data one at a time. For both types, the optical card 1 can have the recorded data read in the manner that the pits 3 have applied laser rays having the quantity less than one used at recording and the reader (not shown) reads out the presence or absence of the pits 3. If defective portions 9 exist on the protection layer 8 or the like, the portions 9 are written or read as incorrect data by laser rays upon writing or reading.
Accordingly, in order to prevent the circulation of the optical recording medium such as optical cards having fatal defects, it is necessary to detect any fatal defects before shipping.
There are two conventional methods for detecting fatal defects existing on the data recording portion 2.
One method is to detect the defective portions 9 on the recording portion 2 by the human sense of sight by using optical means, such as a microscope. This method is a sensory test to which the skilled inspector having a superior ability of inspection is required.
The other method is to detect the defective portions 9 on the recording portion 2 by other than human means, by processing the pattern or signal conversion of the image which is imaged by an image pick up means. This method is based on the principle that the portion 2 of the card 1 as an object of detection is once converted into image data. The detection for defective portions by processing the signal conversion is to output defective detection signals showing the occurrence of the defective portions 9 in the manner that respective portions 2 of two optical cards 1 are imaged and converted into signals, respectively, and both the signals are compared each other so as to detect the defect by the difference in both signals. The detection by pattern processing uses a so-called pattern matching method wherein the imaged pattern of the portion 2 as an object of detection is compared with a standard pattern. If both the patterns have different parts, the defect detection signal is output by regarding it as a defective portion 9.
However, both conventional systems for detecting the defective portions have the following problems.
At first, the defect detection system by the human sense of sight by using the microscope has the problem that the inspection work is complicated because the inspector must work with his hands to detect the defect of cards, and it is necessary to have many inspectors. As the method using this system is a sensory test, the inspector is easily influenced by his fatigue or the like, thereby there is the problem of deteriorating accuracy and reliability of inspection.
Secondly, the first defect detection system using images has a problem in that because an object of comparison is necessary in order to compare both signals, the entire constitution of the system is large-scale and complicated.
Furthermore, the second system using images according to the pattern matching method has a problem that the operation of the system is troublesome because standard patterns are interchanged on all such occasions when the shape or extent of the recording portion as object of detection is changed. As the pattern comparing method requires an actual construction having high accuracy, the manufacturing costs of the entire system increase.